1. Field of Invention
This invention relates to systems and methods to generate diffuse illumination. In particular, this invention is directed to controlling a diffuse light source for a machine vision system.
2. Description of Related Art
Uniform, diffuse illumination of a sample part is often necessary in commercial vision systems to accentuate an edge of the sample part within a designated field of view. Since most sample parts are not transparent, diffuse illumination of the sample part is also necessary so that light which is reflected from the sample part can be collected by an imaging system. Furthermore, an adjustable diffuse illumination source accommodates sample parts having a wide variety of shapes.
Typically, the intensity of light emitted by a light source is adjustable when the magnification of the imaging system is also adjustable. The adjustable illumination provides the ability to illuminate sample parts having different characteristics, such as, for example, shape, composition, and surface finish.
Also, conventional light sources project light onto the sample part at an angle from a plane which is normal to the imaging plane. This angle is referred to as the angle of incidence. Light projected at an angle of incidence which is between 0 and 90 degrees may improve the surface contrast of the image and also more clearly illuminate textured surfaces. Typically, such light sources have a prescribed range for the angle of incidence. Conventionally, the angle of incidence varies between 10xc2x0 and 70xc2x0 relative to the plane that is normal to the optical axis of the imaging system. Such a range is relatively broad and, therefore, provides adequate contrast in an image of a sample part.
Furthermore, conventional vision systems can also adjust the circumferential position of the source of diffuse lighting about an optical axis. Typically, the position of the diffuse lighting source is adjustable in, for example, addressable sectors or quadrants. As such, any combination of sectors and quadrants of such a circular light pattern can be illuminated. Additionally, the intensity level of the light source can be coordinated with the circumferential position of the light source to optimize the illumination of a sample part edge.
For example, some conventional vision systems include an annular light source that emits rectangular or toroidal patterns. The light source is an annulus which is divided into four quadrants. Also, other conventional vision systems include a ring light having an annulus that is subdivided into eight sectors. Additionally, some conventional vision systems have hemispherically-shaped light sources to direct light from a multitude of positions relative to an optical axis. The center of the hemisphere serves as a focal point for the light sources. Furthermore, any combination of sectors and quadrants can simultaneously be illuminated with varying illumination levels.
Recently, manufacturers of conventional vision systems have started offering a solid-state replacement for the traditional tungsten filament lamp, e.g., a halogen lamp, that has been used in conventional diffuse light sources. These manufacturers now offer light emitting diodes (LEDs) that offer higher reliability, a longer service life, greater brightness, lower cost, good modulation capabilities and a wide variety of frequency ranges.
Some manufacturers of such conventional vision systems provide opto-electro-mechanical designs that partially achieve the characteristics of the conventional diffuse light sources discussed above. However, these opto-electro-mechanical devices are complicated, costly, lack versatility, and do not enhance a video inspection process. For example, these light sources require-overly intricate mechanical motion that results in a lower vision system throughput and an increase in cost. Other conventional solid-state light sources require a large number of discrete light sources in a two-dimensional array and an elaborate electronic cross-bar to energize them. Furthermore, other conventional solid-state light sources must accommodate at least fifty discrete light sources in a three-dimensional array housed in a large carriage.
Accordingly, conventional diffuse light sources are incapable of providing a full-featured, reliable, inexpensive system and method to diffusely illuminate a sample part. Moreover, conventional diffuse light sources only marginally provide the capability to control the intensity, angle of incidence and circumferential position. Such conventional diffuse light sources do not optimally illuminate sample parts for dimensional measurements when varying construction (e.g., shape), material (e.g., absorptivity, scattering, etc.), and surface properties (e.g., color or texture) are involved.
This invention provides control systems and methods that achieve the diffuse lighting effects that are currently offered on the market.
This invention separately provides control systems and methods that achieve all these features using a single solid-state source or small number of solid-state sources, such as LEDs or laser diodes.
This invention separately provides control systems and methods that provide color images by assembling RGB images from a monochrome camera.
This invention separately provides control systems and methods that create conventional as well as more versatile diffuse illumination using a simpler, more robust device.
This invention separately provides control systems and methods that allow the selection of illumination color.
This invention separately provides control systems and methods that preserve the high resolution necessary for dimensional metrology measurements without the unnecessary expense of multiple CCD color camera technology.
Using the control systems and methods of this invention, the illumination color may be controlled based on the sample part properties (e.g., pigmentation) in order to improve image contrast. Also, illumination color selection is used to produce a high resolution color image using a monochrome CCD detector.
Exemplary embodiments of the control systems and methods of this invention include a light pattern controller that includes a beam deflector that is mounted on a motor shaft. The beam deflector has a mirror. The beam deflector tilts in proportion to the centrifugal force exerted on the beam deflector when the motor shaft rotates. A light beam incident on the mirror is deflected by an angle that is defined by the tilt of the beam deflector.
Because the beam deflector is rotating, the deflected light beam sweeps out a cone. The deflected light beam cone is incident on a focusing element and sweeps out a circular pattern on the surface of the focusing element. The radius of the circular pattern is dependent on both the distance of the focusing element from the beam deflector and the angle at which the light beam is deflected. The greater the angle of deflection and the farther the focusing element is from the beam deflector, the larger the circular pattern becomes. Therefore, since the rotational speed of the motor shaft is directly proportional to the deflection angle and since the size of the circular pattern is directly proportional to the deflection angle, the size of the circular pattern is directly proportional to the rotational speed of the motor shaft.
The speed at which the light beam traverses the circular pattern is also directly proportional to the rotational speed of the motor shaft. Therefore, the rotational speed of the motor shaft controls both the size of the circular pattern and the speed with which the light beam traverses the light pattern. Thus, the motor and beam deflector control the light pattern.
The light beam is collimated by the focusing element to sweep out a column. This column of light is reflected by a mirror to be substantially parallel to and to surround an optical axis of an imaging device of a vision system. The imaging device, which may include a CCD, employs optical lenses to produce an image of a sample part positioned in a field of view and located at an object plane. The collimated pattern is focused onto the same field of view using another focusing element. Reflected and scattered light from the field of view is imaged onto the CCD using optical lenses.
In other exemplary embodiments of the systems and methods of this invention, the light pattern controller includes a two-dimensional scanning galvanometer. The galvanometer is driven to deflect the light beam to sweep out a cone.
In other exemplary embodiments of the systems and methods of this invention, the light pattern controller includes a liquid crystal device. The liquid crystal device includes an array of addressable sectors that controllably block portions of the light from the light source from impinging on the collimator, or controllably reflect portions of the light from the light source to impinge on the collimator. The liquid crystal device controls the pattern of light from the light source that impinges on the collimator.
The control systems and methods of this invention control the circumferential position, sectors and/or quadrants of the source of diffuse lighting about the optical axis by turning the light source on as the light beam passes a first desired position and by turning the light source off as the light beam passes a second desired position. The position of the effective illumination source is determined by the first and second positions. Moreover, multiple effective illumination sources can be created by turning on and off the light source multiple times for each revolution of the beam deflector. The circumferential length of the sector of illumination is determined based on the amount of time that elapses between the time at which the light source is turned on and the time at which the light source is turned off. This timing is determined either by measuring the rotary speed and position of the motor shaft on which the beam deflector is mounted or by the signals driving the galvanometer. In both cases, pre-registration of the light beam angular location about the imaging system optical axis is known.
Exemplary embodiments of the systems and methods of this invention incorporate the optical source monitoring as described in U.S. patent application Ser. No. 09/220,705, incorporated herein by reference in its entirety. The optical source monitoring of the incorporated 705 application measures the real-time optical power output from the solid-state devices. This is possible on continuous or pulse operated systems. The measurements are taken so that power output variations may be corrected. Power output variations are due primarily to aging, drive current fluctuations and temperature drifts. The intensity measurements permit a level of calibration and instrument standardization which can yield reproducible illumination among an instrument model line.
These and other objects of the invention will be described in or be apparent from the following description of various exemplary embodiments.